Liquid crystal display devices are display devices utilizing a liquid crystal composition for display. The typical display mode thereof is irradiating a liquid crystal panel containing a liquid crystal composition sealed between paired substrates with backlight illumination and applying voltage to the liquid crystal composition to change the alignment of the liquid crystal molecules, thereby controlling the amount of light passing through the liquid crystal panel. Such liquid crystal display devices have features including a thin profile, light weight, and low power consumption, and have therefore been used for electronic devices such as smartphones, tablet PCs, and car navigation systems. The pixel resolution has been increased for uses such as smartphones, which has led to a tendency of an increase in the number of conductive lines and the area of the black matrix disposed in the liquid crystal panel.
In a liquid crystal display device, the alignment of liquid crystal molecules with no voltage applied is typically controlled by alignment films on which an alignment treatment has been performed. The alignment treatment has conventionally been performed by the rubbing method of rubbing the surface of an alignment film with a tool such as a roller. However, since the number of the conductive lines and the area of the black matrix disposed in the liquid crystal panel have been increased, irregularities are now more likely to occur on the substrate surfaces in the liquid crystal panel. With irregularities on the substrate surfaces, the portions near the irregularities may not be properly rubbed by the rubbing method. Such a non-uniform alignment treatment may cause a decrease in the contrast ratio in the liquid crystal display device.
In order to deal with this problem, studies and development have been made on a photo-alignment method which is an alternative alignment treatment method to the rubbing method and irradiates the surface of an alignment film with light. With the photo-alignment method, an alignment treatment can be performed without contact with the surface of the alignment film. The photo-alignment method therefore has an advantage that alignment treatment is less likely to be uneven even with irregularities on a substrate surface, so that a favorable liquid crystal alignment can be achieved on the entire substrate.
Moreover, the increase in the number of conductive lines and the area of the black matrix disposed in the liquid crystal panel may decrease the area ratio of openings usable for display (aperture ratio). Such a decrease in the aperture ratio will be a direct cause of a decrease in the amount of light that can pass through the liquid crystal panel. Significantly increasing the luminance of the backlight has therefore been considered to maintain the display performance, including the contrast ratio, of liquid crystal display devices.
Meanwhile, liquid crystal compositions used for liquid crystal display devices have been desired to have higher stability such that the compositions can withstand the load in the production processes of liquid crystal display devices and contribute to long-term stability of the produced liquid crystal display devices. For example, Patent Literature 1 discloses addition of an antioxidant and a light stabilizer to the liquid crystal composition. Patent Literature 2 also discloses addition of a stabilizer to the liquid crystal composition (see Table C in paragraphs [0208] to [0211]).